This invention relates, in general, to semiconductor memory devices and more particularly, to electrically alterable, nonvolatile floating gate memory devices.
The microprocessor based systems, as well as the related arts, have long required electrically alterable read only memory (EAROM) elements that were nonvolatile and many such devices have, to some extent, filled this need. However, as the computer arts have become more complex in nature and have required high speeds and greater capacity there now exists the need for a high density memory device that may be easily programmed or "written" and, as the occasion arises, to reprogram ("erase" and "rewrite") the device in the field. To this end, devices are presently available to the design engineers that exhibit nonvolatile characteristics but, as will be discussed, the have inherent shortcomings that are overcome by the subject invention.
One such device is found in the family of Floating Gate Avalanche Metal Oxide Semiconductor (FAMOS) devices. The advantage of this type of device resides in the fact that it is independent of any outside current to maintain the stored information in the event power is lost or interrupted. Since these devices are independent of any outside power there is also no need to refresh the device which feature results in a significant savings in power.
The floating gate family of devices usually has source and drain regions of a given conductivity type, formed in a substrate of the opposite conductivity type, at the surface thereof. Between the source and drain regions, and on the surface of the substrate, a gate structure is formed by first applying a thin insulating layer followed by a conductive layer (the floating gate) followed by a second insulating layer in order to completely surround the floating gate and insulate it from the remainder of the device. A second conductive layer (usually referred to as the control gate) is formed over the second insulating layer (in the region of the floating gate) to complete the gate structure. Once such device is exemplified in U.S. Pat. No. 3,500,142 which issued to D. Kahng on Mar. 10, 1970 represents one of the early attempts to achieve nonvolatility.
The major drawback of the prior art devices resides in the fact that high fields are required to produce the necessary avalanche breakdown in order for charge to be placed on the floating gate. Further, to erase charge placed on the floating gate, certain of the prior art devices were provided with a transparent window so that the chip may be flooded with energy in the ultra violet or x-ray portion of the spectrum. Thus, it is extremely difficult to erase a single "word" without erasing all the charge on the device then requiring that the entire chip be completely reprogrammed. Further, the erasing step required an extremely long period of exposure time, of the order of about 30 to 45 minutes, with the device or chip removed from the equipment.
In recent years, the art has progressed to the point where nonvolatile, floating gate read only memory devices have been produced which are electrically alterable. One such memory cell has been described in detail in an article entitled "16-K EE.sup.2 PROM Relies on Tunneling for Byte-Erasable Program Storage" by W. S. Johnson, et al., ELECTRONICS, Feb. 28, 1980, pp. 113-117. In this article the authors describe a "Floating-Gate Tunnel Oxide" structure wherein a cell using a polycrystalline silicon (polysilicon) floating gate structure has its gate member charged with electrons (or holes) through a thin oxide layer positioned between the floating gate and the substrate by means of the Fowler-Nordheim tunneling mechanism. An elevation view of a typical device is described, and shown in FIG. 1 of the article. By using this type of structure, an excessively high floating gate-to-substrate capacitance is produced. However, acceptably low "write" and "erase" operations can only be achieved when most of the applied voltage appears across the tunnel region which, in turn, requires that the floating gate-to-control gate capacitance be larger than the floating gate-to-substrate capacitance. Further, to achieve the required distribution of capacitance to produce the acceptable write and erase characteristics, the prior art has resorted to extending both the first and second polysilicon levels over the adjacent field oxide to obtain additional capacitance. The net result is an undesirably large cell.
In a patent application published in the United Kingdom as UK Patent Application GB No. 2,092,378A on Aug. 11, 1982, in the name of Xicor Inc. and entitled "Dense Nonvolatile Electrically-Alterable Memory Device with Substrate Coupling Electrode", the author describes a floating gate nonvolatile memory fabricated with three layers of polycrystalline silicon (poly). A fourth layer, embedded in the substrate surface is used to bias the floating gate during the write, erase, and read operations. The mechanism for programming the device relys on asperities formed on the upper surface of the first poly level (the programming electrode) to inject charge into the second poly level (the floating gate). To erase the charge, asperities on the upper surface of the floating gate are used to induce charge to migrate from the floating gate to an overlying third poly level which functions as the select/erase electrode. The asperities which texture the surface of the poly layers tend to locally intensify the electric fields thereby lowering the effective polycrystalline silicon barrier height. However, this results in a compromise between read effectiveness and write/erase effectiveness. To merely alter the capacitance in order to improve the effectiveness of either the read operation or the write/erase operation will tend to degrade the operation of the other.